First steps toward the discovery of a new generation of veterinary-specific tetracyclines: The case of oxytetracycline degradation products
Mohamed Rhoumaabc, Wassel Zekriab, Marie-Lou Gaucherabc , Maud de Lagardeac, Francis Beaudryd
a Département de Pathologie et Microbiologie, Faculté de Médecine Vétérinaire, Université de Montréal, Saint-Hyacinthe, QC J2S 2M2, Canada
b Groupe de Recherche et d’Enseignement en Salubrité Alimentaire (GRESA), Faculté de Médecine Vétérinaire, Université de Montréal, Saint-Hyacinthe, QC J2S 2M2, Canada
c Centre de Recherche en Infectiologie Porcine et Avicole (CRIPA), Faculté de Médecine Vétérinaire, Université de Montréal, Saint-Hyacinthe, QC J2S 2M2, Canada
d Département de Biomédecine Vétérinaire, Faculté de Médecine Vétérinaire, Université de Montréal, Saint-Hyacinthe, QC J2S 2M2, Canada
Antimicrobial resistance (AMR) is a critical global health issue, posing a serious and growing threat to human, animal, and environmental health worldwide. The misuse of antimicrobials in human and veterinary medicine has been widely recognized as a key driver of the rapid emergence and global spread of AMR. In response, several initiatives have been implemented in both sectors to mitigate AMR prevalence and safeguard the efficacy of currently available antimicrobials. While the discovery of novel antibacterial agents, especially those targeting Gram-negative bacteria (GNB), remains a major global health priority in human medicine, it has not received comparable attention in veterinary medicine. With the exception of the ionophore class, which is specific to veterinary medicine, most antimicrobial classes used in animals are shared with human medicine. Therefore, the development of novel veterinary-specific antimicrobials, particularly oral antimicrobial agents with high bioavailability and minimal impact on the gut microbiota of farm animals, should be strongly encouraged. The aim of the present study was to evaluate the antibacterial activity of three oxytetracycline (OTC) degradation products (4-α-epi-OTC (4-EOTC), α-apo-OTC, and β-apo-OTC), both individually and in combination with OTC, against selected avian pathogenic Escherichia coli (APEC) strains, while assessing OTC degradation in a simulated poultry gizzard. The microdilution method was used to determine the minimal inhibitory concentration (MIC) of OTC, chlortetracycline, tetracycline, 4-EOTC, α-apo-OTC, and β-apo-OTC against 15 (APEC) strains. The fractional inhibitory concentration index (FICI) was calculated to assess the combined effect of OTC with its degradation products. A conjugation assay was conducted to identify the tetracycline resistance gene mediating APEC resistance to OTC and its degradation products. Results showed that while α-apo-OTC and β-apo-OTC lacked detectable antibacterial activity, 4-EOTC retained inhibitory activity against all tested APEC strains. Notably, a synergistic effect was observed between OTC and 4-EOTC (FICI ≤ 0.5). Preliminary evidence indicated that APEC employed the same resistance determinant (tetA) against both OTC and its degradation products. In addition, OTC remained stable under simulated gizzard conditions, indicating that its degradation likely occurs in downstream compartments of the poultry gastrointestinal tract. The detection of antibacterial activity in 4-EOTC represents a key finding of this study, confirming that OTC degradation products are biologically active and may open new avenues for the development of next-generation tetracyclines for veterinary applications.
